Digital processors are used in modern communication devices to perform complex signal processing while adhering to reasonable power and size constraints. For example, in order to transfer information from one radio to another radio, digital signals are converted into analog transmission signals. This conversion process is performed by a digital-to-analog converter (DAC).
The frequency representation of a digital signal consists of an infinite number of replicas of the equivalent baseband analog signal. The replicas, also referred to as image spectra, are separated in the frequency domain by fs, which is the frequency of the digital sampling clock. All but one replica must be suppressed at the output of a digital-to-analog converter (DAC). Otherwise, due to the close frequency spacing of the replicas, non-linear action in an up-conversion mixer or power amplifier will result in inter-modulation distortion in the pass band.
In the frequency representation of the signal, the signal energy that lies between zero and fs/2 is said to be in the first Nyquist zone. The signal energy that lies between fs/2 and fs is said to be in the second Nyquist zone. The third Nyquist zone is between fs and 3fs/2, etc. Conventionally the signal energy in the first Nyquist zone is the desired replica. The higher frequency replicas are suppressed at the DAC output by an analog low-pass filter and the sample-and-hold function of the DAC. However, a low-pass filter used at the output of the conventional DAC does not integrate well onto integrated circuit chips due to the large physical area consumed by the filter and the precision issues related to passive devices.
In an alternative approach, the digital-to-analog conversion and frequency up-conversion can be achieved by using a higher frequency replica at the DAC output instead of the replica in the first Nyquist zone. This approach uses a band pass filter at the DAC output to suppress the other replicas. The band pass filter could be centered at 3fs with a pass band bandwidth equal to fs. This technique is useful when the desired transmit signal is centered at a multiple of the sampling frequency. Also, a sample-and-hold function of the DAC must not suppress the desired higher-frequency replica (like the return-to-zero, or 2-phase return-to-zero).
However, transmitters may need to operate in multiple frequency bands. The digital clock frequency within these transmitters cannot easily be changed to ensure that a high-frequency replica lands exactly at the desired transmit frequency. Therefore, the baseband signal is shifted from DC to some digital IF frequency (fIF) to offset the replicas away from multiples of the clock frequency. The band pass filter at the DAC output must choose only one of the high frequency replicas. If two replicas are closely spaced in the frequency domain then there is a small region for the filter to transition from pass band to stop band. A short transition region requires a highly selective filter, which means the filter must be physically large and complex to design.
Therefore, what is needed is a method and system for performing digital-to-analog conversion that suppresses unwanted image spectra, does not require highly selective filtering, and scalable to other frequencies.